R975 Radial engines

[Grant Bowker]

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See https://www.shell.com/business-custo...uel/avgas.html for more information about AvGas.

[Bob Phillips]

I would like to revive this discussion of radial engines. The forum has covered a wide variety of topics from history and , identification of different engines, to common problems even fuel grades. In post 31 Dave Hebert mentioned that one issue was the failure of the 8 bolts holding on the crankshaft gear, meaning the rear gear case drive was disconnected from the front section. Thats what this new addition is about. I would appreciate input from forum members as I am looking for solutions to this problem. Anyway here is the story.
ABout a month ago, one of our fellow MV enthusiasts had his C4 equipped M18 out for a number of successful drives. At the end of the day when he went to restart it to garage it for the night, it would not start. The starter would run but the engine would not turn over. The engine could be turned using the cooling fan but there was no connection back to front. A magneto was pulled off and the fan rotated but the mag drive did not turn. So the engine had to come out and the rear case removed for a look see. It had, as Dave had pointed out, sheared off bolts otherwise fairly clean and had no apparent collateral damage. The bolts were removed without difficulty and the gear seemed to good enough to reuse. pictures enclosed.

breakage2.JPG breakage1.JPG breakage3.JPG

Note the unusual shape of the sheared bolts, they are essentially a tapered shoulder cap screw with a square driving head on top with holes to put through lock wires. The manual identifies C1 and C4 crankshaft bolts as
"Screw, machine, sq-hd, dld-f/1kg-wire, alloy -S, 5/16-24NF-2 (19/32) x 1 7/32 overall lgh" it is part number G104-0102120 or CO-200439 , also B206663. Not quite sure what all of the description means, can anybody translate ?

Now before I ask you for your respective opinions on how to repair this, lets look at the later -46 engine gears. The Wright R975 started out with much less power than the finished C4 version. It appears that one of the weak links remained the 8 bolts holding on the rear crank gear- a design unchanged since the mid 1930s?? By contrast, the late 1940s designed 975-46 was built much heavier to handle its boosted horsepower. Some of this has been previously referred to as we noted the heavier case construction, heavier rear main bearing etc. If you check out the next photo you will see a rear crankgear for a -46 and the much heavier method of attaching it. While it uses 8 capscrews, they are flat not tapered. The gear/crankshaft have an aligning dowel on one side. Most importantly, there is a heavy metal projection attached to the gear which is drawn into the rear of the crankshaft witha heavy bolt. A LOT stronger than the C1 and C4 version!

[Bob Phillips]

So if I do this correctly you should see two pictures of a C1/C4 type gear out of the engine and without the view obstructed by the camshaft. You will see the gear bolts up tight against the rear of the crank. It has a small projecting ring on the back which helps align it in the crankshaft. No other attachment ot dowels etc. The -46 in the third photo is much heavier.

C1geara.jpg C1gearb.jpg gear2.jpg

So here is the problem. I am unable to locate any of the original type bolts. If you have any or recognize these, or know of a source I would like to hear from you. Have already checked aircraft supply places and engine rebuilders who declined to sell parts.
So option two..given the fact that the gear is designed to use a tapered shoulder bolt, could we not use conventional tapered socket cap screws in grade 8? They are readily available. They should be just as strong. They would not have the square top and tie wire holes but why not use locktite to hold them in? That would prevent any movement or shifting of the bolts, and the only downside I can see is if the bolts were to shear off it might be a pain to get out the broken bolts. What do you the readership think of this suggestion? All opinions welcome.
Another question has to do with the cause of the failure. The owner checked the detached gearcase to see if there were any obvious bearing failures, or increased resistance to account for the breakage. Everything seemed as normal. Could it just be fatigue in 75 year old engine rebuilt 20 years ago? Aside from checking the gear case, cleaning the sump and putting in clean oil in the engine what other things need to be checked out? Again all opinions welcome.

[Bob Phillips]

Cap screws, tapered shoulder and flat style. available in grade 8.

capscrewgrade8a.jpg capscrewsockethead.jpg

[Jordan Baker]

What about using tapered washers? They come in a variety of sizes. This way you could you use a regular grade 8 bolt/cap screw

Drilled for locking wire (dld-f/lkg-wire)

[David Herbert]

I like Jordan's suggestion a lot. The split tapered collet will be squashed down onto the bolt and there would be no scope for relative movement. I think that this is the ideal solution IF there is enough clearance for the slightly taller head that will result.

As for using standard countersunk Allen screws - I think that the standard countersunk, flat top head will be rather larger diameter than will fit into the recesses in the timing gear. If this is so it may be possible to turn down that extra diameter and leave enough parallel portion to drill for a lock wire. Doing this will not compromise the strength of the screw as the modification is just to the outer end of the head. I think that locktite would be a very bad idea as removing them would be near impossible if they did fail.

Another option would be to turn a small taper onto standard cap screws. I think that this could be done without getting too near the bottom of the hexagonal hole (compare with a countersunk screw ?) but the bearing area to the gear might not be big enough.

Of course the other option would be custom made CNC machined bolts to the original design but in a more modern high tensile steel. They don't need airworthiness certification and they are quite simple so it is do-able.

I don't think that these failures are age related but simply that the original design was too fragile. It may be that the bolts that are failing now were not torqued correctly but we have no way to check this. I know of several engines that have failed this way and been repaired "with standard Allen screws" but I don't know how they overcame the taper issue and am not now in touch with the people that did the repairs. I have not heard of one of these repaired engines failing again but that proves nothing.

David

[Maurice Donckers]

The idea behind these screws was that if you cranked the engine over with a hydrolock , these bolts would shear , and not the whole cylinder would be pushed from the crank case , and create more irrepairable damage